This renewal application requests continued support for a diversified, cell bioloical investigation of the mechanism of cell cleavage. Ultrastructural and biochemical analysis of contractile ring microfilaments is proposed, based upon (1) in situ characterization in cultured vertebrate cells, and (2) isolation of contractile rings from marine eggs. Physiological requirements for cleavage contractility (contractile ring function) will be examined by controlled microperfusion of extracted "models" of cleaving cultured cells, recorded cinemicrographically and by electron microscopy. The role of cytosol calcium in cell cleavage will be investigated by (1) calcium localization histochemistry, and (2) drug-mediated manipulations of cleavage contraction using calcium ionophores and other agents known to modify calcium metabolism.